An image display device such as a plasma display panel (hereinafter, referred to as “PDP”) and a digital mirror device, that performs binary control of emission and non-emission, typically uses a subfield method to implement intermediate gradation display. The subfield method uses a plurality of subfields weighted with the number or amount of emission to divide a single field by temporal decomposition, thereby performing binary control of each pixel for each subfield. In other words, each subfield has its given brightness weight, and the sum of the brightness weights for emitting subfields determines the gradation level.
FIG. 19 illustrates an example configuration of a subfield in a PDP. In this example, a single field is divided into eight subfields (SF1, SF2, . . . , and SF8), where respective subfields have brightness weights (1, 2, 4, 8, 16, 32, 64, and 128). Each subfield is composed of initialization period T1 during which initialization discharge is performed, address period T2 during which data for emission or non-emission is written for each pixel, and sustain period T3 during which pixels with emission data being written are made to emit light all at once. Combining these subfields in various ways for emitting light allows displaying 256-level gradation “0” through “255.” Gradation level “7,” for example, is presented by emitting SF1, SF2, and SF3 having brightness weights 1, 2, and 4, respectively; gradation level “21,” by SF1, SF3, and SF5 having brightness weights 1, 4, and 16, respectively.
In such an image display device that uses the subfield method for displaying multilevel gradation, it is known that false contour noise (hereinafter, referred to as “dynamic false contours”) appears and deteriorates the image quality when displaying motion pictures. (Refer to “False Contour Noise Found in Displaying Motion Pictures by Pulse-width Modulation,” The Institute of Television Engineers of Japan Technical Report, Vol. 19, No. 2, IDY95-21, pp. 61-66. (in Japanese))
Hereinafter, a description is made for the dynamic false contours. Here, a single field is also assumed to be divided into eight subfields (SF1 through SF8), respectively weighted with (1, 2, 4, 8, 16, 32, 64, and 128). As shown in FIG. 20, a case is described where image pattern X moves on the screen of PDP 33 horizontally. Image pattern X has region P1 with gradation level “127” and region P2 with “128.” FIG. 21 is a view in which image pattern X is developed to subfields, where the horizontal axis corresponds to the horizontal position on the screen of PDP 33; the vertical axis, to elapsed time. Further, the hatched areas in FIG. 21 show non-emitting subfields.
When image pattern X is stationary as shown in FIG. 21, a viewer's viewpoint is also fixed to screen position A, and thus pixel-original gradation levels “127” and “128” are perceived. However, when image pattern X moves to the left, the viewpoint also moves to the direction of screen position B-B′, and thus the non-emitting subfields in regions P2 and P1 are viewed. Consequently, gradation level “0”, namely a dark line, is perceived. Reversely, when image pattern X moves to the right, the viewpoint also moves to the direction of screen position C-C′, and thus emitting subfields in regions P1 and P2 are seen, where gradation level “255,” namely a bright line, is perceived. In either case, the gradation levels are largely different from the original (“127” or “128”), and thus are perceived as contours. In this way, dynamic false contours occur where pattern information (hereinafter, referred to as “emission pattern information”) that shows whether a pixel is emitted or not for each subfield largely changes, although the gradation level slightly changes. For example, if subfields weighted as above-mentioned are used, also in cases where the gradation levels of adjacent pixels are “63” and “64,” “191” and “192,” or the like, dynamic false contours are prominently observed, causing the image quality to deteriorate.
Under the circumstances, a method of suppressing dynamic false contours is proposed in Japanese Patent Unexamined Publication No. 2000-276100, for example. That is, convert the gradation level of an image signal to a “first gradation level” where dynamic false contours are unlikely to occur, and to its “intermediate gradation level” by means of a gradation limiting circuit, and then use an error diffusion processing circuit for diffusing an error caused by the conversion to the surrounding pixels, to interpolate skipping of gradation levels. Next, if the converted gradation level is “intermediate gradation level,” round it up or down to the nearest “first gradation level.” Repeat rounding-up and rounding-down alternately by pixel, by line, and by field to present averagely “intermediate gradation levels.”
However, such a method has the following problems. That is, if a part where gradations have some gradient, such as an unfocused part of the image, moves at a speed visually traceable, very large dynamic false contours are observed. Inversely, attempting to suppress the dynamic false contours near a gradation level at which they occur, the number of gradation levels requires to be limited, causing image quality to deteriorate.